Create a conversion factor from "There are 6.02 × 10^23 atoms in 1 gram of hydrogen:"
6.02*10^23 atoms
-------------------------
1 gram
Then mult. 800 grams by this conversion factor:
6.02*10^23 atoms 800 grams
------------------------- * ----------------
1 gram 1
This comes out to 4816*10^23 atoms, which, in scientific notation, is
4.816*10^26 atoms.